Biochemical studies of taste sensation. I. Binding of 14 C-labeled sugars to bovine taste papillae.
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Biochemical studies of taste sensation. XI. Isolation, characterization and taste ligand binding activity of plasma membranes from catfish taste tissue.
Plasma membranes were isolated from taste receptor-containing epithelium of the channel catfish, Ictalurus punctatus. The membranes were prepared by ultracentrifugation of a sedimentable fraction in sucrose, using either a discontinuous density gradient or a continuous linear density gradient. The plasma membranes were characterized by their increased content of 5'-nucleotidase and by electron microscopy, as well as by a greatly diminished content of NADH-cytochrome c reductase and succinate-cytochrome c reductase. The recovery of binding activity for taste ligands was low, because of the long time-period required for ultracentrifugation, but of the recovered activity 80% occurred in the plasma-membrane preparation. Binding of seven chemostimulatory amino acids was demonstrated and found to correspond reasonably well with earlier binding data obtained using a less pure sedimentable fraction. The data provide direct evidence supporting the long-standing hypothesis that taste receptor sites are localized to the plasma membranes.
Amino Acids↗
Photoaffinity labeling of thaumatin-binding protein in monkey circumvallate papillae.
Thaumatin I is an intensely sweet-tasting protein. It was photo-crosslinked with taste papillae of crab-eating monkey by using a conjugated photo-affinity reagent [3H]azidobenzoylthaumatin I. Serial sections of SDS-polyacrylamide gel electrophoresis of the 0.1 M sodium phosphate buffer-soluble fraction from taste papillae had a large peak of radioactivity at the Mr region of approx. 70,000; fractions from non-taste papillae did not. Excess unlabeled thaumatin I reduced the photo-crosslinking at the 70 kDa region; acetylated thaumatin I (which is not sweet) did not. The results show that taste papillae of the monkey contain a protein of Mr approx. 50,000, which binds to thaumatin I (Mr 22,209) but not to completely acetylated thaumatin I. The possibility that the thaumatin-binding protein is a sweet receptor protein is discussed.
Acetylation↗
Characteristics of phosphatidic acid-containing lipoproteins which selectively inhibit bitter taste: high affinity to frog tongue surface and hydrophobic model membranes.
In previous studies (Katsuragi and Kurihara (1993) Nature 365,213--214; Katsuragi et al. (1995) Pharm. Res. 12,658--662) we showed that a lipoprotein composed of phosphatidic acid (PA) and beta-lactoglobulin (LG) selectively suppressed the taste responses to bitter substances without affecting those to other taste stimuli in the frog and man, while complexes composed of other lipids except for phosphatidylserine and LG had little inhibitory activity. In the present study, we found that the lipoproteins having inhibitory activity are adsorbed on the frog tongue surface, while those having no inhibitory activity are not adsorbed. We also examined adsorption of the lipoproteins on model lipid membranes coated on a quartz-crystal microbalance by measuring changes in its frequency. The lipoproteins having inhibitory activity were well adsorbed on the hydrophobic lipid membranes, while the lipoproteins having no inhibitory activity were little adsorbed on the membranes. It seems that receptor sites for bitter substances on the taste cell membranes are hydrophobic and those for other taste stimuli such as salts, acids and sugars are hydrophilic. Hence, the binding of PA-LG to hydrophobic sites of the receptor membranes will lead to selective inhibition of bitterness.
Adsorption↗
Adenylate cyclase responses to sucrose stimulation in membranes of pig circumvallate taste papillae.
1. Typical adenylate cyclase (AC) responses to guanine nucleotides were found in membranes of pig circumvallate (CV) taste papillae. 2. Sucrose stimulated AC activity in the CV membranes and this stimulation was GTP dependent and tissue specific. 3. The stimulatory effect of sucrose in the CV membranes was dependent on the concentration of membranes used in the AC assay. 4. This study provides the first biochemical data on cellular transduction of taste in the pig, compares positively to preliminary results in cattle and supports recent suggestions for a role of cAMP in sweet taste transduction.
Adenylyl Cyclases↗
CDRK and DRK1 K+ channels have contrasting localizations in sensory systems.
Molecular cloning of mammalian potassium channels has revealed an extensively heterogeneous superfamily of potassium channels derived from four basic subfamilies, Shaker, Shaw, Shal and Shab, each with multiple members. The families were first identified in Drosophila, in which subfamily heterogeneity is derived by alternative splicing, while in mammals mainly distinct genes give rise to channel subtypes. Further diversity of mammalian potassium channels is demonstrated by the identification of some which do not belong to any of the four main subfamilies. Although potassium channels are differentiated into fast-inactivating and delayed rectifier types, differential functions of the many mammalian potassium channels are unclear. Moreover, potassium channels function as homotetramers, though in principle heterotetramers might have a physiological role as is the case with heteromers of neurotransmitter receptor subunits. Insight into differential functions of potassium channels may be provided by their regional and subcellular localizations. In the rat brain in situ hybridization and immunohistochemistry have revealed distinct regional localizations for various subfamilies. In one instance a particular subfamily predominated in cell bodies and another in axons. We demonstrated dramatically different localizations for two members of the Shab subfamily, circumvallate papilla delayed rectifier K+ channel (CDRK) and delayed rectifier potassium channel 1 (DRK1), which in major portions of their sequences display more than 90% amino acid identity. In a number of brain regions they occur in distinct neuronal cell types or subcellular compartments, with CDRK predominantly localized diffusely over soma and in fibers and DRK1 most evident in soma and dendritic process.
Animals↗
Glucose-sensitive neurons of the globus pallidus: II. Complex functional attributes.
The globus pallidus (GP) is intimately involved in regulation of various aspects of hunger- and thirst-motivated behaviors. Our parallel neurochemical studies demonstrated the existence of GP neurons whose discharge rates are suppressed by glucose applied microelectrophoretically. In the present series of experiments, we aimed to provide complex, feeding-associated functional characterization--similar to that previously accomplished in the case of lateral hypothalamic and amygdaloid chemosensitive neurons--of these glucose-sensitive (GS) and the glucose-insensitive (GIS) pallidal cells. To do so, extracellular single neuron activity of the GP was recorded in anesthetized rats and anesthetized or awake rhesus monkeys by means of carbon fiber, multibarreled glass microelectrodes during: a) microelectrophoretic administration of chemicals, b) gustatory, and c) olfactory stimulations. In alert primates, activity changes were also recorded during presentation of food and nonfood objects as well as during the performance of a conditioned, high fixed-ratio bar-press feeding task. The half of pallidal cells examined showed firing rate changes during phases of the conditioned alimentary task. In both species, about 1/7 of all neurons tested proved to be GS, while the proportion of cells responding to gustatory and olfactory stimulations was 19% and 16%, respectively. Task-related and taste- and smell-responsive units were mainly found among the GS neurons of the pallidum. These data, along with previous findings, indicate that chemosensitive cells of the GP, in an apparent overlap with units of the central gustatory representation, are involved in a hierarchically organized glucose-monitoring neural network, through which pallidal neurons exert their integrative functions in the central feeding control.
Animals↗
Proton currents through amiloride-sensitive Na+ channels in isolated hamster taste cells: enhancement by vasopressin and cAMP.
Amiloride has been suggested to inhibit responses to a variety of taste stimuli, including salty, sweet, and sour (acid). To test for the involvement of amiloride-sensitive Na+ channels in the transduction of acid stimuli, fungiform taste receptor cells were examined using patch-clamp techniques. Approximately one-half of all cells had amiloride-sensitive Na+ currents (INa) with a Ki value near 0.2 microM amiloride. After blocking voltage-gated conductances, cells having amiloride sensitivity were tested for responses to acid stimuli. Over three-fourths of cells showed an inward proton current (IH+) with an extrapolated reversal potential near approximately +150 mV, which was completely blocked by amiloride (30 microM). Treatment of isolated taste cells with arginine8-vasopressin caused equivalent increases in both INa and IH+; each effect was mimicked by 8-Br-cAMP. Taken together, these results indicate that protons permeate amiloride-sensitive Na+ channels in hamster fungiform taste cells and contribute to acid transduction.
8-Bromo Cyclic Adenosine Monophosphate↗
Cellular basis of taste reception.
The recent application of precise biochemical and electrophysiological techniques to studies of taste cells has brought new insights into the cellular mechanisms of taste transduction. They have revealed that taste cells use a variety of mechanisms for transduction, including apically located ion channels, ligand-gated channels, and receptors coupled to second messenger systems.
Amino Acids↗
Surfactant-like material on the chemoreceptorial surface of the frog's taste organ: an ultrastructural and electron spectroscopic imaging study.
Tannic acid treatment was used to study the morphology of surfactant-like material (SLM) in the taste organ of Rana esculenta and the relation between this material and the cell types of the organ. On the surface of the taste organ SLM was associated with the apical processes of wing and putative taste cells. In SLM, a biphasic pattern was visible, a portion showed a lamellar periodicity (the repeating period of lamellae approximated 45 A), and a second portion showed an homogeneous electron density. Electron spectroscopic imaging revealed the presence of phosphorus and a large amount of calcium associated with the SLM. The result of our work suggests that SLM has a role in the perireceptorial events in the gustatory transduction by concentrating calcium in specific sites of the chemoreceptorial surface.
Animals↗
The nature of taste receptor sites.
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Animals↗
Associations between oral sensation, dietary behaviors and risk of cardiovascular disease (CVD).
We hypothesize that variation in oral sensation influences chronic disease risk by impacting dietary behaviors. Bitterness of 6-n-propylthiouracil (PROP) and fungiform papilla (FP) number serve as genetic taste markers. Data support that nontasters (who taste PROP as least bitter or have lowest FP number) show dietary behaviors that increase CVD risk (e.g. higher alcohol intake, greater preference for and intake of high-fat and sweet foods) and have greater measured CVD risk (e.g. higher blood pressure, less favorable serum lipids). Taste genetics interacts with environmental factors (e.g. taste-related pathologies) to affect oral sensation, dietary behaviors and disease risk. The generalizability of oral sensory and CVD risk relationships has begun to be tested on diverse samples.
Cardiovascular Diseases↗
The sixth taste?
Five taste qualities are recognized in humans: sweet, bitter, sour, salty, and umami. Rats and some other species may also have a sixth taste. Behavioral and electrophysiological data suggest that rats can taste polysaccharides derived from starch. Furthermore, the tastes of sugars and polysaccharides appear to differ in quality. Rats also discriminate different types of polysaccharide and starch molecules. Recent studies indicate that sweet taste is mediated by a T1R2 and T1R3 receptor complex but the identity of the hypothesized polysaccharide taste receptor remains to be established.
Animals↗
An electrophysiological study into the effect of neurotrophin-3 on functional recovery after lingual nerve repair.
OBJECTIVE: Neurotrophin-3 (NT-3) is known to ameliorate central changes that result from peripheral nerve injury and may promote regeneration of myelinated axons. We have assessed its role in the functional recovery of sensory afferents and autonomic efferents after repair of the chorda tympani and lingual nerves in the cat. DESIGN: Six months after entubulation repair, with or without the incorporation of NT-3 at the repair site, the recovery of secretomotor and vasomotor efferents was determined by recording salivary flow from the submandibular gland and temperature changes on the tongue surface, each evoked by stimulation of the repaired nerve. Electrophysiological recordings from the lingual and chorda tympani nerves proximal to the repair allowed characterisation of mechanosensitive, thermosensitive and gustatory afferents. RESULTS: When compared with data from uninjured control animals, both repair groups showed persistent reductions in conduction velocity, receptor sensitivity, spontaneous discharge, proportion of gustatory and thermosensitive units, and rate of salivary secretion. Comparisons between the two repair groups revealed that in the NT-3 group, salivary secretion rate was lower and the activity evoked in the chorda tympani by gustatory or thermal stimuli was lower, but the spontaneous discharge rate was higher. Mechanosensitive units in the lingual nerve had slower conduction velocities but the mechanoreceptive field size, adaptation time and discharge frequency had increased. CONCLUSIONS: Despite its known trophic role in the lingual somatosensory system, NT-3 did not enhance functional recovery from injury and had a negative effect on the long-term outcome for sensory and autonomic fibres.
Action Potentials↗
Hypothalamic histamine release by taste stimuli in freely moving rats: possible implication of palatability.
Our previous study indicated that taste information via the chorda tympani (CT) activates the central histaminergic system in anesthetized rats. However, the physiological roles of taste-induced histamine release remain unknown, thus to further investigate the relationship between histamine release and gustatory information, in the present study we investigated the effect of taste stimuli infused intraorally on histamine release using in vivo microdialysis in free moving rats. Consistent with findings from our previous study, application of NaCl and HCl caused significant increases in histamine levels further supporting the suggestion that this phenomenon is attributed to the excitation of the CT. When rats were intraorally infused with quinine HCl (QHCl) solution, a significant increase in hypothalamic histamine release was observed. On the other hand, histamine release was decreased by sucrose and saccharin solutions. When rats were conditioned to acquire taste aversion to sucrose solution or saccharin solution, instead of the histamine decrease seen by the palatable solutions, the pattern of histamine release was similar to that seen by QHCl solution. From these observations, it is concluded that the histamine release by the infusion of these tastants may be explained by two mechanisms-by causing a transient increase after taste stimulation and by causing a decrease relative to the tastant's palatability.
Animals↗
Extracellular proton sensing of the rat gustatory cyclic nucleotide-gated channel.
Elevations of the intracellular levels of cyclic nucleotides appear to cause the cation influx through gustatory cyclic nucleotide-gated (CNGgust) channels expressed in taste cells. Although changes in the oral pH may directly regulate the activity of the CNGgust channel, the mechanism of pH-dependent control of the channel is not understood. In the present study, we combined the whole-cell patch-clamp recording and the site-directed mutagenesis to investigate the effect of extracellular pH on the ion permeation through CNGgust channels expressed in HEK293 cells. Extracellular acidification strongly inhibited ion permeation through open CNGgust channels. Mutation of Glu(289) remarkably attenuated the pH-dependence of the channel, suggesting that Glu(289) in the pore-forming region is a major proton acceptor site. However, the mutant E289A-CNGgust channel possesses the other residual protonation/deprotonation site. The channel activity, tightly regulated by pH(o) and [cNMP](i), suggests the involvement of its pH(o)-dependent ion permeation in taste signal transduction events.
Animals↗
The human taste receptor hTAS2R14 responds to a variety of different bitter compounds.
The recent advances in the functional expression of TAS2Rs in heterologous systems resulted in the identification of bitter tastants that specifically activate receptors of this family. All bitter taste receptors reported to date exhibit a pronounced selectivity for single substances or structurally related bitter compounds. In the present study we demonstrate the expression of the hTAS2R14 gene by RT-PCR analyses and in situ hybridisation in human circumvallate papillae. By functional expression in HEK-293T cells we show that hTAS2R14 displays a, so far, unique broad tuning towards a variety of structurally diverse bitter compounds, including the potent neurotoxins, (-)-alpha-thujone, the pharmacologically active component of absinthe, and picrotoxinin, a poisonous substance of fishberries. The observed activation of heterologously expressed hTAS2R14 by low concentrations of (-)-alpha-thujone and picrotoxinin suggests that the receptor is sufficiently sensitive to caution us against the ingestion of toxic amounts of these substances.
Bicyclic Monoterpenes↗
Gustatory processing: a dynamic systems approach.
Recent gustatory studies have provided a growing body of evidence that taste processing is dynamic and distributed, and the taste system too complex to be adequately described by traditional feed-forward models of taste coding. Current research demonstrates that neuronal responses throughout the gustatory neuroaxis are broad, variable and temporally structured, as a result of the fact that the taste network is extensive and heavily interconnected, containing modulatory pathways, many of which are reciprocal. Multimodal influences (e.g. olfactory and somatosensory) and effects of internal state (e.g. attention and expectation), shown in both behavioral and neuronal responses to taste stimuli, add further complexity to neural taste responses. Future gustatory research should extend to more brain regions, incorporate more connections, and analyze behaviors and neuronal responses in both time- and state-dependent manners.
Animals↗
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